IS-INSTANCE-VALUE
=================
syntax: (is-instance-value
<evaluates-to relation-name>
<evaluates-to instance-spec>
<lisp-type: T>
[:read-dependents
{T | NIL | Var}]
[:if-undetermined
{T | NIL | Var}])
This function checks, whether there is a particular
value in the value set of the instance relation. If
':if-undetermined' is not set to NIL, then a
'if-undetermined-action' will be started before the
relation value is found. If ':read-dependents' is not
set to NIL, then the read-dependents will be informed
after the relation value is found.
example: >(is-instance-value 'place louvre Berlin)
NIL
>(is-instance-value 'place louvre Paris)
#<capital-city Paris>